The change of his personality should be after Zhang Miao defected. Speaking of Zhang Miao, many people who play in the Three Kingdoms will feel very proud, but the relationship between this person and Cao Cao is actually very iron. Yuan Shao, Cao Cao and Zhang Miao were all good friends when they were young. These three people are rich second generation, good at fencing and chivalry, and should hit it off. Later, Yuan Shao asked Cao Cao to kill Zhang Miao, but Cao Cao refused personally. He said that Meng Zhuo was his relative and friend, and he wouldn't do such a thing. The rise of Cao Cao was in the territory of Zhang Miao. When Cao Cao just levied Xuzhou, he told his family that if he couldn't come back, he would go to Zhangmiao. Dong Zhuoxi fled, Cao Cao wanted to pursue, and other governors stood by. Only Zhang Miao helped Cao Cao's army.
Cao Cao killed Cai Mao by mistake.
All these show how strong the relationship between two people is. But Zhang Miao finally betrayed Cao Cao at the instigation of Chen Gong and others. As for why Zhang Miao betrayed Cao Cao, I'm afraid this is mainly related to Yuan Shao. When the allied forces begged Dong, Zhang Miao was hated by Yuan Shao for accusing him of arrogance. Yuan Shao asked Cao Cao to kill Zhang Miao, but Cao Cao refused. At that time, although Zhang Miao felt the virtue of Cao Cao, there was obviously a shadow in his heart. Since then, the growing strength of Yuan Shao and a series of joint military actions between Cao Cao and Yuan Shao in the north have made Zhang Miao in a weak position even more uneasy, fearing that Cao Cao would eventually hurt himself because of Yuan Shao. It was this fear that finally betrayed Cao Cao and ushered in Lu Bu. In the end, his choice turned into a tragedy.